1. Field of the Invention
The present invention relates to a traction-drive type driving-force transmission mechanism capable of transmitting a driving force or reducing a rotational speed, through means of a traction force generated between rollers, and an image forming apparatus equipped with the traction-drive type driving-force transmission mechanism.
2. Description of the Related Art
As a conventional technique relating to traction-drive type driving-force transmission mechanisms, there has been known a mechanism which comprises a sun roller coupled to a rotary shaft, a plurality of planetary rollers arranged around the sun roller, and a carrier supporting respective planetary shafts of the planetary rollers (see, for example, JP 2000-329206A, which will hereinafter be referred to as “document D1”). The mechanism disclosed in the document D1 is designed such that each of the planetary rollers is rotated through means of a traction force generated between the sun roller and each of the planetary rollers, and a movement of each of the planetary rollers orbited around the sun roller along a pair of orbit rings is transmitted from the carrier to an output shaft.
In order to generate a sufficient traction force between the sun roller and each of the planetary rollers, it is necessary to strongly press the planetary roller against the sun roller (i.e., bring the planetary roller into press contact with the sun roller) in the presence of a lubricant while rotatably supporting the planetary roller by the corresponding planetary shaft. For this purpose, in the mechanism disclosed in the document D1, the planetary shaft and the planetary roller are formed and arranged to ensure a certain gap therebetween, and the pair of orbit rings are disposed to sandwich therebetween the planetary roller in such a manner that an inclined surface formed in an inner periphery of each of the orbit rings is brought into press contact with an inclined portion (inclined surface) formed on each of axially opposite lateral sides of the planetary roller, to allow the planetary roller to be pressed against an outer peripheral surface of the sun roller.
In the mechanism disclosed in the document D1, each of the planetary rollers is adapted to be freely displaceable relative to the planetary shaft within the range of the gap, although the planetary shaft is fixed to the carrier. Thus, when the orbit rings are brought into press contact with the respective inclined surfaces of the planetary rollers, the planetary rollers will be strongly pressed against the sun roller, so that a required traction force can be generated in the presence of a lubricant.
However, the mechanism disclosed in the document D1 requires using the two orbit rings made of a costly material, such as tool steel, and the resulting increase in material cost will lead directly to an increase in product cost. Moreover, due to an increase in the number of components requiring high-accuracy assembling, such as the orbit rings, an assembling process becomes complicated, which leads to a problem about an increase in production cost.
Further, in the mechanism disclosed in the document D1, the planetary shaft and the planetary roller are arranged to define a gap therebetween. This technique involves a possibility that a rotation axis (rotation central axis) of the planetary roller is displaced at a certain amount of angle with respect to an axis (central axis) of the planetary shaft. Thus, when the planetary roller is pressed against the sun roller, an outer peripheral surface of the planetary roller is likely to be slightly inclined relative to the outer peripheral surface of the sun roller. In this case, a pressure distribution on the outer peripheral surface of the planetary roller becomes uneven in an axial direction of the planetary roller, and thereby so-called “edge contact” occurs to cause uneven wear (i.e., partial or local wear) on the outer peripheral surface of the sun roller.
Further, the planetary rollers in the document D1 are built in a casing-side subassembly including the output shaft and a pressing member (orbit rings) but not in a drive-side subassembly including the sun roller. Thus, in an operation of assembling the casing-side subassembly and the drive-side subassembly, the sun roller is inserted into the casing-side subassembly to bring the outer peripheral surface thereof into contact with the respective outer peripheral surfaces of the planetary rollers, and then further slidingly moved inside the casing-side subassembly while maintaining the contact. This operation is also likely to cause scratch or abrasion on the sun roller and the planetary rollers. Thus, the technique disclosed in the document D1 involves a problem of being unable to obtain a long-lifetime traction-drive type driving-force transmission mechanism.